1. Field of the Invention
The present invention relates to a loop antenna, and particularly to a loop antenna making use of a ground conductor plate for inducing image charge to enhance radiation efficiency.
2. Description of the Related Art
For emerging wireless transmission devices, dimensions of antennas have great effects on wireless transmission and must be in the order of the radio wavelength for transmission efficiency. However, in some bandwidths, radio wavelengths are much longer than the length of antenna that wireless transmission devices can afford, such that radiation efficiency is very low. In order to improve radiation efficiency, It is necessary to use complicated antennas and RF circuits. That will causein the wireless system in high cost, low yield, and high power consumption. Thus, the benefits of wireless transmission are lost. To improve radiation efficiency, it is necessary to make use of ground conductor plates within wireless devices and polarization of signal source of an antenna.
Most conventional loop antennas are magnetic dipoles. When the conventional loop antennas are using near a ground metallic plate, their radiation efficiencies will be reduced by the ground metallic plate.
A block diagram of a wireless mouse with a conventional magnetic dipole loop antenna is shown in FIG. 1. RF module 220 includes an amplifier 520, a phase lock loop circuit 450, a filter 470, and a voltage control oscillator 480. The base band circuit includes a CPU 420, a shift encoder 300, a memory 440 such as non-volatile memory EEPROM, and a switch 550. The first end of the loop antenna 240 is coupled to the signal end of the amplifier 520. The second end of the loop antenna 240 is coupled to the ground end of the amplifier 520.
When the wireless mouse operated, the CPU 420 reads out the channel frequencies, the sampling frequencies of the photo detector 310, and the identification code from the memory 440. The identification code identifies different wireless mice in the same transmission region and the same transmission frequency. For a same computer, each wireless mouse has a unique identification code. When the wireless mouse is powered up, the memory 440 records and updates the peripheral identification code of the computer.
The CPU 420 controls the channel frequencies by controlling the modulation frequency by the phase lock loop circuit 450. The CPU reads the data of the transmission channel frequency from memory 440, and sends the data to the phase lock loop circuit 450 to generate the carrier signal of the transmission channel. The user can use the channel select key 500 to select the transmission channel from the memory 440.
The CPU 420 provides a determined information to modulator 560 to modulate the transmitted signal. The modulator 560 comprises a voltage control conciliator (VCXO) in series with a crystal to generate a reference frequency and uses this frequency to work as a FSK modulator. The modulator 560, the phase lock loop circuit 450, the filter 470, and the voltage control oscillator form a feedback loop which generates a RF carrier signal with precise frequency. The RF carrier signal is fed into the circular loop antenna 241 through the amplifier 520. The modulated reference frequency of the modulator 560 is generated by switching over resonance capacitors of the reference oscillator. The reference frequency is changed by the resonance capacitor that is FSK modulation. The signals of switching over is the encode data of the mouse operation. The filter must have enough bandwidth to track the modulation of the reference frequency.
When the wireless mouse used on a metallic table which acting as a ground conductor plate, that causes cancellation of the magnetic dipole source. The input impedance of the loop antenna 240 changes, that will shorten the transmission distance of the wireless mouse. A diagram of the loop antenna 240 and the ground conductor plate 230 is shown in FIG. 2. The wireless mouse is used on the ground conductor plate 230. The first end of the loop antenna is coupled to the signal end of the amplifier 520, and the second end of the loop antenna 240 is coupled to the ground end of the RF module 220. The loop antenna 240 is using upon the ground conductor plate 230 and parallels closely to the ground conductor plate 230. The current of the loop antenna 240 is parallel to the ground conductor plate 230. Owing to good conduct characteristic of the ground conductor plate 230, the current of the loop antenna 240 induces an image current distribution in the ground conductor plate which makes the tangential electric field is zero. The image magnetic dipole source caused by the image current is opposite to the magnetic dipole source in the current of the loop antenna 240, as shown in FIG. 3. Therefore, the image magnetic dipole reduced the radiation intensity of the loop antenna 240. Usually, the wireless mouse is used on the surface of a table. The distance between the wireless mouse and the table is small. Thus, as the desk-top of the table is made by conduct plate 230, when the distance between the loop antenna 240 and the ground conductor plate 230 is smaller the effect of reduced radiation intensity is more significant.
It is necessary to design an antenna system not only reduced but also enhanced radiation intensity. It is also necessary to take advantage of a conductor plate when a wireless transmission device is using on it.
It is therefore an object of the present invention to provide a loop antenna enhanced by an environment with a ground conductor plate.
To achieve the above objects, the present invention provides a loop antenna system. According to the embodiment of the invention, the loop antenna system includes a ground conductor plate coupled to a ground end and a loop antenna having a helix wire wounded on a toroid. The helix wire has a first end coupled to the ground end. The toroid has a principal axis AX1 and a minor axis AX2. The principal AX1 is perpendicular to the ground conductor plate. The minor AX2 is parallel to the ground conductor plate.
When the radio wavelength of the transmission signal is beyond the dimensions of the loop antenna, the magnetic current is distributed along the minor axis AX2, and the electric dipole is along the principal axis AX1 and perpendicular to the ground conductor plate.